The regulation of epidermal hyperplastic growth is central to an understanding of epidermal regeneration, carcinogenesis and epidermal proliferative diseases such as psoriasis. The production of an epidermal hyperplasia is associated with a massive increase in ribosomes, due largely to ribosome synthesis, which returns to normal levels during epidermal regression. Increases in ribosome numbers indicates that there must be large increases in protein synthesis. It becomes important, therefore, to determine the rates of protein synthesis during epidermal hyperplastic growth, and to relate them to the rates of ribosome synthesis. We propose to measure the rates of protein synthesis during the production and regression of epidermal hyperplasia produced by abrasion or the application of the tumor promoter TPA. Epidermal hyperplastic growth is the result of the interplay of cell proliferation and cell cornification (keratinization). Therefore, to give meaning to the changes in ribosome and protein synthesis during epidermal hyperplastic growth, we must relate them to the changes in epidermal cell proliferation and cornification. We propose to determine the rates of epidermal cornification by measuring the changes in the transit time during the production and regression of an epidermal hyperplasia produced by abrasion or the application of TPA. The rates of epidermal cell proliferation will be determined throughout epidermal hyperplastic growth, produced by abrasion or by the application of TPA by determining the 3H-thymidine labelling index, the length of the cell cycle, and the growth fraction.